1. Field of the Invention
The present invention relates to an optical semiconductor apparatus for use in optical communication systems and the like, for example, the optical semiconductor apparatus may be used in oscillation polarization mode selective semiconductor lasers which can be driven by a direct modulation method with reduced dynamic wavelength fluctuation even during high-speed modulation operation, and in polarization-insensitive optical amplifiers which can substantially equally amplify any polarized light component. The present invention also relates to a driving method for modulating or driving the optical semiconductor apparatus. The present invention also relates to a light source apparatus and to an optical communication system which use the optical semiconductor apparatus as a transmitter, for example.
More particularly, the present invention relates to an oscillation polarization mode selective semiconductor laser whose oscillation polarization mode can be switched between mutually-perpendicular polarization modes. The semiconductor laser of the present invention can suppress dynamic wavelength fluctuation even during high-speed modulation and stably achieve high-density frequency or wavelength division multiplexing (FDM or WDM) optical communication and the like.
2. Related Background Art
As an oscillation polarization mode selective dynamic single mode semiconductor laser, the following device has been developed and proposed. The oscillation polarization mode selective device has a structure that can be modulated by a digital signal which is produced by superposing a minute-amplitude digital signal on a bias injection current. The device is a distributed feedback (DFB) laser in which a distributed reflector of a grating is introduced into a semiconductor laser resonator or cavity and wavelength selectivity of the grating is utilized. In the device, strain is introduced into an active layer of a quantum well structure, or its Bragg wavelength is located at a position shorter than a peak wavelength of a gain spectrum, so that gains for transverse electric (TE) mode and transverse magnetic (TM) mode are approximately equal to each other for light at wavelengths close to an oscillation wavelength, under a current injection condition near an oscillation threshold. Further, a plurality of electrodes are arranged and currents are unevenly injected through those electrodes. An equivalent refractive index of the cavity is unevenly distributed by the uneven current injection, and oscillation occurs in one of the TE mode and the TM mode and at a wavelength which satisfies a phase matching condition and takes a minimum threshold gain. When the balance of the uneven current injection is slightly changed to vary a competitive relation of the phase condition between the TE mode and the TM mode, the oscillation polarization mode and wavelength of the device can be switched.
In that semiconductor device, an antireflection coating is provided on one end facet to asymmetrically employ effects of the uneven current injection into its output-side portion and its modulation-electrode portion. Alternatively, lengths of the electrodes are made different from each other to introduce a structural asymmetry.
Furthermore, Japanese Patent Laid-Open No. 2-117190 discloses a semiconductor laser apparatus in which two semiconductor devices are arranged serially or in parallel. One of devices principally oscillates or amplifies a wave in a predetermined polarization mode, and the other one chiefly oscillates or amplifies a wave in another polarization mode. Those devices are formed in a common layer or in parallel layers.
However, the above-discussed conventional oscillation polarization mode selective DFB semiconductor laser, which selects the oscillation polarization mode depending on the phase condition, is sensitive to the phase at the end facet. As a result, the oscillation wavelength and polarization mode of the device depend on the current injection condition in a complicated fashion, and fluctuation in characteristics of the oscillation polarization mode and the like appears among individual devices. If antireflection coatings were provided on both end facets to solve those disadvantages, asymmetry in a light propagation direction of the device would be weakened and the effect of the uneven current injection would be reduced. Thus, stable switching of the oscillation polarization mode would be lost.
In the apparatus of Japanese Patent Laid-Open No. 2-117190, the oscillation or amplification of the wave in a predetermined polarization mode is performed by selecting a device's geometric shape, as a result of which yield is decreased depending on fluctuation in etched depth and ridge width that occurs during a ridge fabrication process.
Furthermore, in a fabrication process of a semiconductor laser device, the device or crystal is generally degraded during a process in which the wafer is exposed to atmosphere. Moreover, during repetitive crystal growth steps, V-group element, whose vapor pressure is high, is likely to escape from the substrate surface when the substrate temperature is raised prior to a crystal growth step thereon, and hence the crystal is degraded.